The invention relates generally to circuit test procedures and, more particularly, to techniques for detecting defects in circuits at a macroscopic level.
As part of the digital circuit manufacturing process, tests are commonly performed to detect defects within the fabricated circuits. Two types of defects that can occur in digital circuits are xe2x80x9cstuck atxe2x80x9d defects and delay defects. A xe2x80x9cstuck atxe2x80x9d defect is presently in a circuit when a portion of the circuit that is supposed to transition between logic values (e.g., from logic zero to logic one, and vice versa) during circuit operation does not transition. That is, the circuit portion is xe2x80x9cstuck atxe2x80x9d either a logic one value or a logic zero value. A delay defect is present when a circuit portion can transition between logic values, but the transition occurs either before or after it is supposed to. As can be appreciated, both types of defect can have a devastating effect on circuit performance and circuits having such defects should normally be identified and discarded. Delay defects are generally detected by operating a circuit at full speed while monitoring the transition activity at selected nodes in the circuit. A known test sequence is applied to the circuitry that should generate transitions at the selectable nodes within predetermined time windows. If one or more of the transitions occur outside of the corresponding time window, the circuit is deemed to have a delay defect.
As is well known, the speed at which digital circuits operate is continually increasing. Thus, to continue to perform defect tests at full operational speed in the future, higher performance test equipment will be needed. However, higher performance test equipment is typically expensive and can significantly increase the overall cost of the circuit manufacturing process. As can be appreciated, it is generally desirable to keep manufacturing costs down to increase the competitiveness of manufactured products in the marketplace by minimizing price to the consumer.
Therefore, there is a need for a method and apparatus for detecting defects within manufactured circuitry that does not require full speed operation of a device under test.